1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a thermally driven, monolithic, ink-jet printhead having a nozzle plate that is formed integrally with a substrate and a hydrophobic coating layer formed on a surface of the nozzle plate, and a method for manufacturing the same.
2. Description of the Related Art
In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume ink droplet of a printing ink at a desired position on a recording sheet. Ink-jet printheads are largely classified into two types depending on the ink droplet ejection mechanisms: a thermally driven ink-jet printhead, in which a heat source is employed to form and expand a bubble in ink thereby causing an ink droplet to be ejected, and a piezoelectrically driven ink-jet printhead, in which a piezoelectric crystal bends to exert pressure on ink, thereby causing an ink droplet to be expelled.
An ink droplet ejection mechanism of the thermally driven ink-jet printhead will now be described in detail. When a pulse current flows through a heater formed of a resistive heating material, heat is generated by the heater to rapidly heat ink near the heater to approximately 300° C. Accordingly, the ink boils and bubbles are formed in the ink. The formed bubbles expand and exert pressure on the ink contained within an ink chamber. This causes a droplet of ink to be ejected through a nozzle from the ink chamber.
The thermally driven ink-jet printhead may be further subdivided into top-shooting, side-shooting, and back-shooting types depending on the direction of ink droplet ejection and the direction in which a bubble expands. The top-shooting type refers to a mechanism in which an ink droplet is ejected in a direction that is the same as a direction in which a bubble expands. The back-shooting type is a mechanism in which an ink droplet is ejected in a direction opposite to the direction in which the bubble expands. In the side-shooting type, the direction of ink droplet ejection is perpendicular to the direction in which the bubble expands.
Thermally driven ink-jet printheads need to meet the following conditions. First, a simple manufacturing process, low manufacturing cost, and mass production must be provided. Second, to produce high quality color images, a distance between adjacent nozzles must be as small as possible while still preventing cross-talk between the adjacent nozzles. More specifically, to increase the number of dots per inch (DPI), many nozzles must be arranged within a small area. Third, for high-speed printing, a cycle beginning with ink ejection and ending with ink refill must be as short as possible. That is, the heated ink and heater should cool down quickly to increase an operating frequency. Fourth, heat load exerted on the printhead due to heat generated by the heater must be small, and the printhead must operate stably under a high operating frequency.
FIG. 1A illustrates a partial cross-sectional perspective view of a structure of a conventional thermally driven printhead. FIG. 1B illustrates a cross-sectional view of the printhead of FIG. 1A for explaining a conventional process of ejecting an ink droplet.
Referring to FIGS. 1A and 1B, a conventional thermally driven ink-jet printhead includes a substrate 10, a barrier wall 14 disposed on the substrate 10 for defining an ink chamber 26 filled with ink 29, a heater 12 installed in the ink chamber 26, and a nozzle plate 18 having a nozzle 16 for ejecting an ink droplet 29′. If a pulse current is supplied to the heater 12, the heater 12 generates heat and a bubble 28 is formed due to the heating of the ink 29 contained within the ink chamber 26. The formed bubble 28 expands to exert pressure on the ink 29 contained within the ink chamber 26, thereby causing an ink droplet 29′ to be ejected through the nozzle 16. Then, the ink 29 flows from a manifold 22 through an ink channel 24 to refill the ink chamber 26.
The process of manufacturing a conventional top-shooting type ink-jet printhead configured as above involves separately manufacturing the nozzle plate 18, which includes the nozzle 16 and the substrate 10, which includes the ink chamber 26 and the ink channel 24, and bonding them together. The manufacturing process is complicated and misalignment may occur during the bonding of the nozzle plate-18 and the substrate 10. Furthermore, since the ink chamber 26, the ink channel 24, and the manifold 22 are arranged on a same plane, there is a restriction on increasing the number of nozzles 16 per unit area, i.e., the density of nozzles 16. This restriction makes it difficult to implement a high printing speed, high-resolution ink-jet printhead.
Recently, in an effort to overcome the above problems of conventional ink-jet printheads, ink-jet printheads having a variety of structures have been proposed. FIG. 2 illustrates an example of a conventional monolithic ink-jet printhead.
Referring to FIG. 2, a hemispherical ink chamber 32 and a manifold 36 are formed on a front surface and a rear surface of a silicon substrate 30, respectively. An ink channel 34 is formed at a bottom of the ink chamber 32 and provides communication between the ink chamber 32 and the manifold 36. A nozzle plate 40, including a plurality of passivation layers 41, 42, and 43 stacked on the substrate 30, is formed integrally with the substrate 30.
The nozzle plate 40 has a nozzle 47 formed at a location corresponding to a central portion of the ink chamber 32. A heater 45 connected to a conductor 46 is disposed around the nozzle 47. A nozzle guide 44 extends along an edge of the nozzle 47 toward a depth direction of the ink chamber 32. Heat generated by the heater 45 is transferred through an insulating layer, which is the lowermost passivation layer 41, to ink 48 within the ink chamber 32. The ink 48 then boils to form bubbles 49. The formed bubbles 49 expand to exert pressure on the ink 48 contained within the ink chamber 32, thereby causing an ink droplet 48′ to be ejected through the nozzle 47. Then, the ink 48 flows through the ink channel 34 from the manifold 36 due to surface tension of the ink 48 contacting the air to refill the ink chamber 32.
A conventional monolithic ink-jet printhead configured as above has an advantage in that the silicon substrate 30 is formed integrally with the nozzle plate 40 thereby simplifying the manufacturing process and eliminating the chance of misalignment. Another advantage is that the nozzle 46, the ink chamber 32, the ink channel 34, and the manifold 36 are arranged vertically to increase the density of nozzles 46, as compared with the conventional ink-jet printhead shown in FIG. 1A.
In a conventional ink-jet printhead, since ink is ejected as an ink droplet, the ink must be ejected in a discrete ink droplet form to provide acceptable printing performance. In an ink-jet printhead, a size, a shape, and a surface property of the nozzle greatly affect a size of the ejected ink droplet, a stability of the ink droplet ejection, and an ejection speed of the ink droplet. In particular, the surface property of the nozzle plate greatly affects the characteristic of the ink ejection.
In the ink-jet printhead shown in FIG. 2, the passivation layers 41, 42, and 43 formed around the heater 45 are formed using low heat conductive insulating materials, such as oxide or nitride, for purposes of providing electrical insulation. Thus, a considerable amount of time is required for the heater 45, the ink 48 within the ink chamber 32, and a nozzle guide 44, all of which are heated during the ejection of the ink 48, to sufficiently cool and return to an initial state, thereby making it difficult to increase the operating frequency to a sufficient level.
In the ink-jet printhead shown in FIG. 2, since the nozzle plate 40 is relatively thin, it is difficult to secure a sufficient length of the nozzle 47. A small length of the nozzle 47 not only decreases the directionality of the ink droplet 48′ ejected but also prohibits stable high-speed printing since the meniscus in the surface of the ink 48 after ejection of the ink droplet 48′ retreats into the ink chamber 32. In an effort to solve these problems, the conventional ink-jet printhead has a nozzle guide 44 formed along the edge of the nozzle 47. However, if the nozzle guide 44 is too long, this not only makes it difficult to form the ink chamber 32 by etching the substrate 30 but also restricts expansion of the bubbles 49. Thus, the use of the nozzle guide 44 causes a restriction on sufficiently securing the length of the nozzle 47.